Polyesters have been produced commercially on a large scale for processing into shaped articles such as fibers, primarily from poly(ethylene terephthalate). Synthetic polyester yarns have been known and used commercially for several decades, having been first suggested by W. H. Carothers, U.S. Pat. No. 2,071,251, and then Whinfield and Dickson suggested poly(ethylene terephthalate) in U.S. Pat. No. 2,465,319.
Although many polyester polymers (including copolymers) have been suggested, the polyester most widely manufactured and used hitherto for textile fibers has been poly(ethylene terephthalate), which is often referred to as homopolymer PET. Homopolymer PET has generally been preferred over copolymers because of its lower cost, and also because its properties have been entirely adequate, or even preferred, for most end-uses. It is known, however, that homopolymer PET requires special dyeing conditions (high temperature requiring super-atmospheric pressure) not required for nylon fibers, for example, so copolyesters have been suggested and used commercially for some purposes. Homopolymer PET is often referred to as 2G-T, while poly(trimethylene terephthalate) is referred to as 3G-T (although some have started calling this PTT), and poly(tetramethylene terephthalate) is referred to as 4G-T, and so on. Some interest has been shown in 3G-T, and also in 4G-T, but 2G-T is the polyester polymer that has so far been used the most, so is discussed mostly hereinafter, but it will be understood that the invention is expected to apply also to other polyesters, for instance other C.sub.2 -C.sub.4 alkylene terephthalates, such as 3G-T and 4G-T mentioned above, and copolyesters.
Polyester fibers are either (1) continuous filaments or (2) fibers that are discontinuous, which latter are often referred to as staple fiber or cut fibers, and are made by first being formed by extrusion into continuous polyester filaments, which are processed in the form of a tow of continuous polyester filaments before being converted into staple. An important stage in the processing of such continuous polyester filaments has been "drawing" to increase the orientation of the long chain polyester molecules, and thereby improve the properties of the filaments. The present invention relates to improvements in this drawing stage and to the improved products resulting therefrom.
Mostly, the objective of synthetic fiber producers has been to replicate advantageous properties of natural fibers, the most common of which have been cotton and wool fibers.
Most of the polyester cut fiber has been of round cross-section and has been blended with cotton. Recently, however, U.S. Patents Nos. 5,591,523 (DP-6255) and 5,626,961 (DP-6365-A) and copending application Ser. No. 08/662,804 (DP-6400) filed Jun. 12, 1996, and now allowed, corresponding respectively to WO 97/02372, WO 97/02373 and WO 97/02374, the disclosures of which are hereby incorporated herein by reference, have disclosed inventions relating to polyester tows that are suitable for conversion to slivers on a worsted or woollen system and downstream processing on such systems, eventually into fabrics and garments. The present invention has been made in the course of that work, so is described with particular reference to its value in drawing polyester filaments in tows for further processing in such systems, but is not confined to drawing such tows and is believed to have potential for use more broadly when drawing other bundles of polyester filaments.
As, for example, has been disclosed in U.S. Pat. No. 5,591,523 (DP-6255), filaments of different denier per filament (dpf) have sometimes been desired, so surprise was expressed in Example 1 of that patent that it was possible to spin undrawn polyester filaments that had been spun of significantly different denier on the same spinning machine without adjusting the natural draw ratio and then subsequently to draw an intimate mixture of these spun filaments simultaneously in the same tow at the same draw ratio to provide filaments with excellent properties that were different because of their differing dpfs (col 6, lines 15-29). The present invention expands on this surprising finding and extends it to the drawing simultaneously of other mixed filament bundles beyond the mixed filament tows specified in that patent.